1. Field of invention
The present invention relates to machines for measuring dimensions on machined parts. In particular, the invention is a system for reducing the friction in mechanical bearings on a coordinate measuring machine.
2. Description of prior art
Some coordinate measuring machines of prior designs have a moveable carriage mounted to a base. The carriage is exclusively supported in one of the orthogonal directions by a mechanical bearing or an air bearing.
Mechanical bearings have the disadvantage that some friction is inherent in the bearings and this friction is approximately proportional to the total load on the mechanical bearings. The prior systems which have a carriage exclusively supported in one direction by mechanical bearings have the disadvantage of the inherent friction.
Air bearings have been suggested for supporting completely the carriage in at least one direction. An air bearing system requires a rather wide air pad (to obtain the necessary lift) and an equally wide support for the air pad, generally in the form of a rail. The rail must be machined to highly precise dimensions and extend the measuring length of the machine. Because of the larger width of the precision straight rail (compared to the width of a rail for supporting a mechanical bearing) and the long length, the machining costs for producing such a rail are quite high.
The known prior art air bearing systems generally have a rigid coupling between the air bearing pad and the carriage of the measuring machine. This is disadvantageous, in that the bearings, which ride on a bearing surface, transmit every irregularity in the bearing surface to the measuring machine, causing measurement error.